The team looked at information available about the 1918 flu pandemic, which killed between 50 million and 100 million people globally in the space of about 18 months.

Some research has shown that on average it took a week to 11 days for people to die - which fits in more with the known pattern of a bacterial infection than a viral infection, write Klugman and colleagues.

"We observed a similar 10-day median time to death among soldiers dying of influenza in 1918."

People with influenza often get what is known as a "superinfection" with a bacterial agent. In 1918 it appears to have been Streptococcus pneumoniae.

"Neither antimicrobial drugs nor serum therapy was available for treatment in 1918," Klugman's team write.

Today there are vaccines that protect against different strains of S. pneumonia, which cause infections from pneumonia to meningitis.

Worst-case scenario

Most health experts believe that another pandemic of influenza is inevitable. There were smaller pandemics in 1958 and in 1967.

Many government projections have been based on a worst-case 1918 scenario, in which tens of millions of people would die globally and up to 40% of the work force would be out for weeks, either sick, caring for others who are sick, or avoiding public places for fear of infection.

"Based on 1918 we would project less mortality in an era of antibiotics," says Klugman.

"We are currently modelling this, assuming of course that the bacterial superinfections remain susceptible to the antibiotics and that sufficient antibiotics are available."

A pandemic occurs when a new strain of flu begins infecting people. One fear is that H5N1 influenza, currently infecting birds in Asia, Europe and Africa, might make the jump to people.

H5N1 has infected people a relatively small number of people resulting in 254 deaths since 2003, according to the World Health Organization.

Every year seasonal influenza kills between 250,000 and 500,000 people.